patch 8.2.3372: line2byte() value wrong when adding a text property

Problem:    line2byte() value wrong when adding a text property. (Yuto Kimura)
Solution:   Adjust length for text property. (closes #8772)  Also fix it for
            deleting a line.
diff --git a/src/testdir/test_textprop.vim b/src/testdir/test_textprop.vim
index 373df6e..f21fd40 100644
--- a/src/testdir/test_textprop.vim
+++ b/src/testdir/test_textprop.vim
@@ -857,6 +857,15 @@
   call assert_equal(1491, line2byte(401))
   bwipe!
 
+  new
+  call setline(1, range(520))
+  call assert_equal(1491, line2byte(401))
+  call prop_add(2, 1, {'type': 'comment'})
+  call assert_equal(1491, line2byte(401))
+  2delete
+  call assert_equal(1489, line2byte(400))
+  bwipe!
+
   call prop_type_delete('comment')
 endfunc